1. Field of the Invention
The present invention relates to a method for estimating deterioration of the optical member and the maximum pulse number that can be applied to the optical members, or optical lenses, such as illumination lenses and projection lenses for use in KrF (248 nm) or ArF (193 nm) excimer laser lithography and optical lenses for excimer laser processing apparatus or the like.
2. Discussion of the Related Art
For the photolithographic technology for exposing and transcribing fine patterns of integrated circuits onto wafers made of silicon, or the like, an exposure apparatus called a stepper has been used. Due to an increasing trend towards higher integration of LSI, the wavelength of the light source of the stepper has been becoming shorter and shorter; from the g-line to the i-line, and further to KrF and ArF excimer lasers. Conventional general-use optical glass can no longer be used for the illumination system or the projection lens of this type of excimer laser stepper. Thus, silica glass and fluorite have been used for the optical members of the excimer laser exposure apparatus, for example.
The silica glass and fluorite used for lenses of the illumination and projection systems processing excimer laser beams require, as in the case of conventional optical glass, a bulk transmittance of 99.5%/cm or more, or 99.8%/cm or more. Considerable amounts of research have been conducted to develop optical members having such a high transmittance in the ultraviolet range. Among others, optical deterioration in optical members; i.e., changes in optical properties of optical members over time caused by the properties of excimer laser light, such as short wavelengths and flashing nature of the laser beams (so-called solarization and compaction), has significant effect on the imaging performances. Therefore, this deterioration has been one of the major problems in developing suitable glass members, and it is important to investigate the behavior and causes of this optical deterioration.
In particular, in order to investigate the behavior of the deterioration of silica glass for use with an irradiation energy of 10 mJ/cm.sup.2 per pulse or less, it is most desirable to measure changes in physical properties, such as the transmittance, refractive index, and the surface profile, after actually irradiating a test piece with a laser beam of an actual energy density. However, with such a low irradiation energy density, each change in the physical properties due to one irradiation pulse is extremely small. Therefore, in order to produce detectable changes in these physical properties, it is necessary to increase the number of irradiation pulses. (In other words, the measurement period needs to be extremely long.) However, in reality, because of limited manpower and economic and temporal restrictions, it is difficult to keep on measuring one sample for several years. In addition, in the cases where stable performances need to be guaranteed over a decade or so, as in the case of reduction projection lenses made of expensive, special silica glass, such measurement is even more difficult.
Given these circumstances, there is a strong demand for developing a method of providing an accurate estimation of the life-span (estimation of durability) without actually measuring the deterioration (changes in the physical properties) in optical members over several years. Conventional estimation methods, which purport to achieve this goal, are effective only when the relationship between the reduction of the transmittance and the total number of irradiating pulses are linearly related. Thus, the conventional estimation methods are only applicable to estimation of the durability for a very low energy range, as may often be the case for the durability test of projection lenses. However, for illumination systems, when changes in transmittance, which occur due to irradiation of pulses of 1.times.10.sup.9 pulses or more at an energy density of several mJ/cm.sup.2 per pulse to several tens of mJ/cm.sup.2 per pulse, are in question, there is a non-negligible possibility that the reduction in transmittance may be grossly overestimated. Therefore, it has been difficult to accurately estimate the life-span of the optical members.
In addition, in order to estimate the life-span of an optical system of excimer laser steppers or apparatus processing excimer laser beams in general, it is necessary to derive reliable formulae representing changes in the properties of the lens material irradiated with the excimer laser. Accordingly, it is necessary to experimentally investigate the precise causes of the deterioration of silica glass or other optical member used in excimer laser optical member. Nonetheless, the published technical articles so far only show partial or qualitative results on this point, and very little research has been conducted to determine the reliable estimation formulae; concrete formulae and/or methodology, which can quantitatively provide reliable estimation, have never been presented.
Furthermore, in the case that the optical member is silica glass, if the transmittance decreases by 3%/cm or more, it has been assumed that this deterioration in optical performances of the silica glass be caused by heat. Thus, when it was necessary to prove that there is no deterioration due to heat, it was necessary to select a test sample having a transmittance reduction of 3%/cm or less.